The present invention relates to an improved echo canceler for use with drivers of varying clock rates.
Echo cancellation is known per se. For example, in a speaker phone, echo cancellation prevents sound that is emitted from an omni-directional speaker and captured by a nearby microphone from returning to the signal's source and interfering with communication. Consider an example where a first party speaks to a second party located at the speaker phone. When the first party speaks, the party's voice is broadcast from the speaker phone. Not only is the first party's voice heard by the second party, but the voice also is captured by the speaker phone's microphone. The voice signal reflects off of various surfaces, for example walls, ceilings, furniture and people. The reflected signal is captured by the microphone at some time delayed from the time that the signal was emitted by the speaker. If the reflected signal is not eliminated from the aggregate input signal, the reflected speech signal may be perceived as an annoying echo when delivered to the first party.
Echo cancelers, as the name implies, eliminate the echo generated by the reflected signals. Typically, they do so by buffering a copy of the output audio signal at the speaker phone. The echo canceler monitors the input signal from the microphone and identifies when and how the reflected signal appears in the input signal. When a reflected signal is identified, the processor generates an inverted replica of the reflected signal from the buffered signal and applies it to the input signal. When applied in a correct timing relationship, the replica cancels the reflected signal.
Echo cancelers appear in a variety of applications beyond merely speaker phones. For example, they may be used in video conferencing equipment. In all known echo cancelers, the output speaker equipment and the input microphone equipment are driven by a single clock source. The single clock source permits the correct timing relationship to be maintained between the buffered output signal (the source of the replica) and the captured input signal. Speaker and microphone equipment are not driven by independent clocks because drift among them would prevent the echo canceler from establishing and maintaining the correct timing relationship between the replica and reflected signals.
It is anticipated that computer systems such as personal computers and/or network computers may include hardware that enables telecommunication or video conferencing. However, such computer systems may provide independent speaker and microphone equipment, each with it own clock. Even if the clocks had the same ideal clock rate, echo cancellation heretofore could not be provided for such a system because drift among the two clocks would impair the operation of the echo canceler. Echo cancellation certainly could not be provided for a system where speaker and microphone equipment possessed independent clocks with different clock rates.
Accordingly, there is a need in the art for an echo canceler that is suitable for use with independently clocked input and output devices.